Continuously variable transmissions differ from the conventional automatic and manual transmissions in that continuously variable transmissions do not employ fixed gear ratios. Instead, they employ variably spaced sheave plates and hydraulic systems for the control thereof whereby the two sheaves are independently pressurized to clamp a belt at any desired effective sheave ratio. The effective gear ratio is adjusted by opening and closing the sheaves and forcing the belt radially inward or outward. Because of this mode of varying the effective gear ratio, prior art arrangements of the engine, damper, and starting clutch followed by the transmission have been found undesirable. In such systems gear ratio changes are attempted while the clutch is disengaged and the transmission sheaves not in motion, resulting in problems of wear and noise. One proposed solution to this problem is disclosed in U.S. Pat. No. 4,433,594, issued Feb. 28, 1984 and entitled Variable Pulley Transmissions, where the clutch is moved downstream of the transmission and the sheaves are continuously moving.
In manual transmissions, it has been found desirable to couple the output of the torsion damper to a clutch assembly which is in turn connected to the manual transmission whose output is applied to the drive train of an automotive vehicle. Such a system is well known in the art. The torsion damper in such a manual transmission usually has relatively short travel with a high spring rate provided for the take-up of engine vibrations and also high starting and shut-down torques. While the clutch is disengaged, the manual transmission can be shifted to any desired forward or reverse gear to apply the appropriate gear ratio and direction of travel to the subsequent drive train without risk of damage or excessive wear. Excessive torques are avoided by clutch slippage and a multiple rate damper spring system.
With the development of automatic transmissions, early transmissions used a hydraulically actuated torque converter which reduced the need for a vibration damper as engine vibrations and impulses as well as start-up and shut-down transients could be damped hydraulically in the torque converter.
In later developments, a positive drive clutch was interposed in the torque converter to bypass the torque converter at higher speeds and provide a direct drive between the engine and transmission. With that new development in automatic transmission systems, the need for a vibration damper again arose although start-up and shut-down problems were less severe. Such a system could use a conventional short travel vibration damper. This history of transmission system development as it relates to torsional dampers is explained in U.S. Pat. Nos. 4,585,427 and 4,139,995 which are incorporated herein by reference.
Where a torque converter is not utilized or is by-passed in an automatic transmission system by a direct drive clutch the need arose for a damper which would accommodate both engine vibration and the high torque of start-up and shut-down. Such dampers, known as long travel dampers, are also described in U.S. Pat. Nos. 4,139,995 and 4,585,427. Dampers in such systems are required to take up not only engine vibrations but start-up and shut-down torques as well.
With the advent of the continuously variable transmission system of U.S. Pat. No. 4,433,594 and the repositioning of the clutch, the damper was outside of the transmission enclosure and was designed to run dry. However, experience has shown that a dry torsion damper such as disclosed in that patent has several drawbacks when applied to a continuously variable transmission of the general type disclosed in U.S. Pat. No. 4,433,594. Furthermore, as taught in the U.S. Pat. No. 4,433,594, it was found highly desirable in continuously variable transmission systems to place the starting clutch at the output of the transmission rather than at the input. Among the reasons the configuration of this patent was found desirable was the requirement for shifting the effective gear ratio in the continuously variable transmission while the transmission was in motion. Attempts to change the effective gear ratio while the transmission was static resulted in excessive wear, excessive shifting forces and possible damage to the transmission parts during the efforts to change gear ratio under static conditions.
In working with transmission systems of the type provided in U.S. Pat. No. 4,433,594, however, it was found that the use of a torsion damper of conventional design produced unsatisfactory results, excessive wear in such dry dampers and insufficient damping for engine start-up and shut down torques. When the system was designed to have a damper rate appropriate for accommodating the inertia and physical forces experienced in start-up and shut down, the damper was ineffective with respect to vibration from the engine operation or the design parameters were exceeded.